The department will need time to see how it can be repaired. It cannot be determined yet when the slow lane can be opened, but the fast lane may be operating in time for Monday's rush.

Bus and MTR services have been increased to cope with extra passengers.

The blaze is believed to have started during maintenance work on ancient water pipes under the road by 35 Water Supplies Department staff when liquid asphalt ignited. All the workers were moved to safety after the fire broke out about 3.30am yesterday.

The fire happened about 200 meters inside the tube's Kowloon exit and raged across a 60-meter section. Smoke billowed for 400 meters outside.

"There was a lot of smoke and I could not breathe," one worker said. "Somebody yelled that there was

a fire and we just ran for our lives.

"As we reached the exit we could not see anything because of the smoke."

About 140 firemen, 10 breathing- apparatus teams, six jets and 35 fire engines were sent to fight the blaze.

The maintenance work was started in 2009 in an attempt to make the five pipes last 50 more years. The pipes were laid in the 1960s to carry water from the Sha Tin Water Treatment Plant to Kowloon during a drought.

The Lion Rock Tunnel was built only because the colonial government needed to drill a hole in Lion Rock to carry the water pipes.

New Territories East divisional fire commander Leung Kwok-ming said it took firemen a long time to put out the fire mainly because the manholes to the water pipes are only large enough for one person to get in, and it took time to find the seat of the blaze.

"It was very hot down there, with a lot of smoke, and this was why the mission was tough," he said.

There is only one manhole every 60 meters in the tunnel. The fire was put out at around noon.

A Water Supplies Department spokesman said the reason for the blaze cannot be confirmed until a detailed investigation is carried out.

But he could not rule out the possibility of a short circuit or someone smoking a cigarette near the water pipes.

"The pipes are already very old," he said. "The layer of asphalt catches fire very easily, even from a spark."

But he stressed the workers did not need to use flames during the maintenance work. The tunnel operator said there are fire prevention devices inside the tunnel but not under the road near the water pipes.

Meanwhile, about 1,500 disgruntled firemen have voted against the Fire Service Department's 51-hour-week proposal which will reduce manpower and fire appliances during night shifts, saying the new arrangement is putting citizens at risk.

Members of the Fire Services Department Staffs General Association cited concerns over maintaining emergency service quality amid the cutback on the night shift. They are demanding that weekly working hours be cut from the current 54 to 48.

The union will hold another meeting to gauge members' views, and talk to department managers next week.